1. Field of the Invention
An object of the present invention is a trajectory correction device for electron tubes. It can be applied notably to multibeam tubes and, in particular, to klystron type microwave tubes.
2. Description of the Prior Art
FIG. 1 gives a schematic view of a prior art multibeam electron tube.
The structure has a shape generated by revolution around an axis z. Electron beams, six in this example, namely 10, 11, 12, 13, 14, 15, are produced by a means (not shown) and respectively go through holes A, B, C, D, E, F, pierced in a first disk 20, centered on the axis z and placed in a plane (x, y), and holes A', B', C', D', E', F', pierced in a second disk 22, also centered on the axis z and located in a plane (x', y').
Each pair A and A', B and B', C and C', D and D', E and E', F and F', is centered on a straight line parallel to the axis z.
The electrons are guided by a magnetic field, called a main field, which is generated by a system 24 of coils having the axis z as its axis of symmetry and having DC currents flowing through it. The principal magnetic field also takes the axis z as the axis of revolution.
This main magnetic field is essentially directed along the axis z between the two disks 20 and 22, but the axial component Bz of this field varies as a function of the distance from the axis. In other words, the axial component of the field shows a radial gradient.
This non-uniformity of the magnetic field as well as the off-centered position of the beams causes a drift in the trajectory of the electrons.
More precisely, the mean trajectory of the electrons is not directed in parallel to the axis z. This is illustrated in FIG. 2 which gives a schematic view of the disk of FIG. 1, showing the radial and azimuthal drift of an electron beam. Each beam undergoes a radial drift .DELTA.R and an azimuthal drift .DELTA..rho..
As can be seen in FIG. 2, the electron beam 10 tends to strike the disk 22 at A" instead of passing through A'. There is a similar situation for the other beams.
It is known that the radial drift .DELTA.R can be cancelled. It is enough for the fluxes of the field through circles going through the holes A and A' (B and B', C and C', D and D', E and E', F and F' respectively) to be identical.
For the proper functioning of the tube, two conditions are then imposed on the main magnetic field: its amplitude should be substantially the same at the level of the homologous holes A and A', B and B', . . . , and the fluxes through the circles passing by these space should be identical.
However, these tubes designed in this way also have a drawback which is that they have an azimuthal drift, with an amplitude of .DELTA..rho..
An aim of the present invention is to overcome this drawback by providing the means to remove this azimuthal drift.
To this end, the invention proposes the use of coils and/or additional ferromagnetic parts, capable of creating a magnetic correction field, which gets added to the main magnetic field and brings the electrons back to the space A'.